Variable flip angle balanced steady-state free precession for lower SAR or higher contrast cardiac cine imaging

Cardiac cine balanced steady-state free precession (bSSFP) imaging uses a high flip angle (FA) to obtain high blood-myocardium signal-to-noise and contrast-to-noise ratios (CNR). Use of high FAs, however, results in substantially increased SAR. Our objective was to develop a variable FA bSSFP cardiac cine imaging technique with: (1) low SAR and blood-myocardium CNR similar to conventional constant FA bSSFP (CFA-bSSFP) or (2) increased blood-myocardium CNR compared to CFA-bSSFP with similar SAR.

Variable FA bSSFP cardiac cine imaging was achieved using an asynchronous k-space acquisition, which is asynchronous to the cardiac cycle (aVFA-bSSFP). Bloch simulations and phantom experiments were performed to compare the signal, resolution, and frequency response of the variable FA bSSFP and CFA-bSSFP schemes. Ten volunteers were imaged with different aVFA-bSSFP and asynchronous CFA-bSSFP schemes and compared to conventional segmented CFA-bSSFP.

The SAR of aVFA-bSSFP is significantly decreased by 36% compared to asynchronous CFA-bSSFP (1.9 ± 0.2 vs. 3.0 ± 0.2 W/kg, P <  10(-10)) for similar blood-myocardium CNR (34 ± 6 vs. 35 ± 9, P = 0.5). Alternately, the CNR of the aVFA-bSSFP is improved by 28% compared to asynchronous CFA-bSSFP (49 ± 9 vs. 38 ± 8, P <  10(-4)) with similar SAR (3.2 ± 0.5 vs. 3.3 ± 0.5 W/kg, P = 0.6).

aVFA-bSSFP can be used for lower SAR or higher contrast cardiac cine imaging compared to the conventional segmented CFA-bSSFP imaging.